Since the time approximately 100 years ago when first technologies for wireless data transmission began to be employed, the bandwidth available for transmission has grown continuously. As is known, the width of the frequency band that can be used for transmission depends on the carrier frequency, so that as the frequency increases, the transmission bandwidths available also increase. Nowadays, carrier frequencies in the range from a few kilohertz to many gigahertz are used. Thus, so-called “wireless HD” operates with a carrier frequency of 60 GHz and bandwidths of 4 Gbit/s. In order to be able to achieve data rates in the range of 10 Gbit/s and higher, waves in the terahertz range will also be used as carriers in the future.
The first attempts at data transmission with terahertz waves were carried out with pulsed transmitters that employed femtosecond lasers. However, broadband data transmissions require continuous-wave sources, which is to say sources of terahertz radiation that operate continuously. Such continuous-wave THz sources can be constructed from two independent lasers stabilized to one another, for example. Other continuous-wave THz sources use two-color diode lasers, which simultaneously emit two spatially superimposed waves with a spectral spacing in the THz range. Despite all efforts, however, the generation of terahertz waves today is still complicated and expensive, so that the goal is to use the limited available continuous-wave terahertz power for data transmission as efficiently as possible. In addition, it must be noted that terahertz waves are subject to strong attenuation in air, and the radiation is noticeably attenuated over distances of just a few meters even in the spectral windows that are quite transparent for terahertz waves.